...that Hilary Tolkien himself did not lead a very interesting life, even from the point of view of most Tolkien fans. If so, then the 10% of the content that was objectionable may have been the only reason for writing the book in the first place.

A quick internet search turned up the suggestion that "after the 1976 Copyright Law, all such materials—created before 1978 and never published afterwards—come into the public domain after “life of the author + 70 years or December 31, 2002” (whichever come up later)." J. R. R. Tolkien's generation of family died in the 1960s-1980s; his children's generation is mostly alive or very recently passed away. So under this law, any letters written to Hilary Tolkien by family members of his own generation or younger would be protected from reproduction without permission until no earlier than the 2030s.

The distinction that is commonly made in this area is between published and unpublished material. Published material has less protection, because it has already entered the public realm and so the public has some claim on it to balance the author's. But unpublished material was always meant to be private, and so it is protected for a much longer period.